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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 4642

NNTP Extensions Working Group                                 J. Vinocur
Internet Draft                                        Cornell University
Expires: July 2005                                          K. Murchison
                                                      Oceana Matrix Ltd.
                                                               C. Newman
                                                        Sun Microsystems
                                                            January 2005


                          Using TLS with NNTP
                     draft-ietf-nntpext-tls-nntp-04


Status of this memo

     By submitting this Internet-Draft, I certify that any applicable
     patent or other IPR claims of which I am aware have been disclosed,
     and any of which I become aware will be disclosed, in accordance
     with RFC 3668.

     Internet-Drafts are working documents of the Internet Engineering
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     Internet-Drafts are draft documents valid for a maximum of six
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Copyright Notice

     Copyright (C) The Internet Society (2005).

Abstract

     This memo defines an extension to the Network News Transport
     Protocol [NNTP] to provide connection-based security (via Transport
     Layer Security [TLS]).  The primary goal is to provide encryption
     for single-link confidentiality purposes, but data integrity,
     (optional) certificate-based peer entity authentication, and



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     (optional) data compression are also possible.

Table of Contents

     0. Changes from Previous Version ............................  2
     1. Introduction .............................................  3
        1.1. Conventions Used in this Document ...................  3
     2. The STARTTLS Extension ...................................  3
        2.1. Advertising the STARTTLS Extension ..................  4
        2.2. STARTTLS Command ....................................  4
           2.2.1. Usage ..........................................  4
           2.2.2. Description ....................................  4
              2.2.2.1. Processing After the STARTTLS Command .....  6
              2.2.2.2. Result of the STARTTLS Command ............  7
           2.2.3. Examples .......................................  7
     3. Augmented BNF Syntax for the STARTTLS Extension ..........  9
        3.1. Commands ............................................  9
        3.2. Capability entries ..................................  9
     4. Summary of Response Codes ................................  9
     5. Security Considerations ..................................  9
     6. IANA Considerations ...................................... 11
     7. References ............................................... 12
        7.1. Normative References ................................ 12
        7.2. Informative References .............................. 13
     8. Authors' Addresses ....................................... 13
     9. Acknowledgements ......................................... 13
     10. Intellectual Property Rights ............................ 14
     11. Copyright ............................................... 14

0. Changes from Previous Version

     New:
     o  Noted that MODE READER must not be used nor advertised after TLS.

     Changed:
     o  CAPABILITIES replaces LIST EXTENSIONS.
     o  Neither 480 nor 483 are valid responses to STARTTLS.
     o  Removed requirement that the client SHOULD send a CAPABILITIES
        immediately after a successful TLS.
     o  Use common language between this draft and AUTHINFO regarding
        unsolicited use of the extension and resetting of server state and
        caching of info obtained prior to a security layer.
     o  Capabilities are now case-insensitive.
     o  Changed reference to IANA requirements in [NNTP] from Section 8 to
        Section 3.3.4.

     Clarified:
     o  When TLS negotiation begins for both client and server.



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     o  If the server doesn't like the results of TLS, it only sends 483
        response to restricted NNTP commands (e.g. QUIT, HELP, CAPABILITIES
        are excluded).
     o  The MODE READER state change does not get discarded after TLS is
        negotiated.

     Other:
     o  Editorial nits.

     Outstanding issues:
     o  Make sure we correctly reference the text in [NNTP] regarding MODE
        READER use after TLS.

1. Introduction

     Historically, unencrypted NNTP [NNTP] connections were satisfactory
     for most purposes.  However, sending passwords unencrypted over the
     network is no longer appropriate, and sometimes strong encryption
     is desired for the entire connection.

     The STARTTLS extension provides a way to use the popular TLS [TLS]
     service with the existing NNTP protocol.  The current
     (unstandardized) use of TLS for NNTP is most commonly on a
     dedicated TCP port; this practice is discouraged for the reasons
     documented in section 7 of "Using TLS with IMAP, POP3 and ACAP"
     [TLS-IMAPPOP].  Therefore, this specification formalizes and
     extends the STARTTLS command already in occasional use by the
     installed base.

1.1. Conventions Used in this Document

     The notational conventions used in this document are the same as
     those in [NNTP] and any term not defined in this document has the
     same meaning as in that one.

     The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD
     NOT", "MAY", and "OPTIONAL" in this document are to be interpreted
     as described in "Key words for use in RFCs to Indicate Requirement
     Levels" [KEYWORDS].

     In the examples, commands from the client are indicated with [C],
     and responses from the server are indicated with [S].

2. The STARTTLS Extension

     This extension provides a new STARTTLS command and has the
     capability label STARTTLS.




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2.1. Advertising the STARTTLS Extension

     A server supporting the STARTTLS command as defined in this
     document will advertise the "STARTTLS" capability label in response
     to the CAPABILITIES command.  However, this capability is not
     advertised after successful authentication [NNTP-AUTH], nor is it
     advertised once a TLS layer is active (see section 2.2.2.2).  This
     capability MAY be advertised both before and after any use of MODE
     READER, with the same semantics.

     As the STARTTLS command is related to security, cached results of
     CAPABILITIES from a previous session MUST NOT be relied on, as per
     section 11.6 of [NNTP].

     Example:
        [C] CAPABILITIES
        [S] 101 Capability list:
        [S] VERSION 2
        [S] READER
        [S] IHAVE
        [S] STARTTLS
        [S] LIST ACTIVE NEWSGROUPS
        [S] .

2.2. STARTTLS Command

2.2.1. Usage

     This command MUST NOT be pipelined.

     Syntax
        STARTTLS

     Responses

        382 Continue with TLS negotiation
        502 Command unavailable [1]
        580 Can not initiate TLS negotiation

     [1] If a TLS layer is already active, or authentication has
     occurred, STARTTLS is not a valid command (see section 2.2.2.2).

     NOTE: Notwithstanding section 3.2.1 of [NNTP], the server MUST NOT
     return either 480 or 483 in response to STARTTLS.

2.2.2. Description





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     A client issues the STARTTLS command to request negotiation of TLS.
     The STARTTLS command is usually used to initiate session security,
     although it can be used for client certificate authentication
     and/or data compression.

     An NNTP server returns the 483 response to indicate that a secure
     or encrypted connection is required for the command sent by the
     client.  Use of the STARTTLS command as described below is one way
     to establish a connection with these properties.  The client MAY
     therefore use the STARTTLS command after receiving a 483 response.

     If a server advertises the STARTTLS capability, a client MAY
     attempt to use the STARTTLS command at any time during a session to
     negotiate TLS without having received a 483 response.  Servers
     SHOULD accept such unsolicited TLS negotiation requests.

     If the server is unable to initiate the TLS negotiation for any
     reason (e.g. a server configuration or resource problem), the
     server MUST reject the STARTTLS command with a 580 response.
     Otherwise, the server issues a 382 response and TLS negotiation
     begins.  A server MUST NOT under any circumstances reply to a
     STARTTLS command with either a 480 or 483 response.

     If the client receives a failure response to STARTTLS, the client
     must decide whether or not to continue the NNTP session.  Such a
     decision is based on local policy.  For instance, if TLS was being
     used for client authentication, the client might try to continue
     the session, in case the server allows it to do so even with no
     authentication.  However, if TLS was being negotiated for
     encryption, a client that gets a failure response needs to decide
     whether to continue without TLS encryption, to wait and try again
     later, or to give up and notify the user of the error.

     After receiving a 382 response to a STARTTLS command, the client
     MUST start the TLS negotiation before giving any other NNTP
     commands.  The TLS negotiation begins for both the client and
     server with the first octet following the CRLF of the 382 response.
     If, after having issued the STARTTLS command, the client finds out
     that some failure prevents it from actually starting a TLS
     handshake, then it SHOULD immediately close the connection.

     Servers MUST be able to understand backwards-compatible TLS Client
     Hello messages (provided that client_version is TLS 1.0 or later),
     and clients MAY use backwards-compatible Client Hello messages.
     Neither clients or servers are required to actually support Client
     Hello messages for anything other than TLS 1.0.  However, the TLS
     extension for Server Name Indication [TLS-EXT] SHOULD be
     implemented by all clients; it also SHOULD be implemented by any



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     server implementing STARTTLS that is known by multiple names
     (otherwise it is not possible for a server with several hostnames
     to present the correct certificate to the client).

     Although current use of TLS most often involves the dedication of
     port 563 for NNTP over TLS, the continued use of TLS on a separate
     port is discouraged for the reasons documented in section 7 of
     "Using TLS with IMAP, POP3 and ACAP" [TLS-IMAPPOP].

2.2.2.1. Processing After the STARTTLS Command

     After the TLS handshake has been completed, both parties MUST
     immediately decide whether or not to continue based on the
     authentication and privacy achieved (if any).  The NNTP client and
     server may decide to move ahead even if the TLS negotiation ended
     with without authentication and/or without privacy because NNTP
     services are often performed without authentication or privacy, but
     some NNTP clients or servers may want to continue only if a
     particular level of authentication and/or privacy was achieved.

     If the NNTP client decides that the level of authentication or
     privacy is not high enough for it to continue, it SHOULD issue a
     QUIT command immediately after the TLS negotiation is complete.  If
     the NNTP server decides that the level of authentication or privacy
     is not high enough for it to continue, it SHOULD either reject
     subsequent restricted NNTP commands from the client with a 483
     response code (possibly with a text string such as "Command refused
     due to lack of security"), or reject a command with a 400 response
     code (possibly with a text string such as "Connection closing due
     to lack of security") and close the connection.

     The decision of whether or not to believe the authenticity of the
     other party in a TLS negotiation is a local matter.  However, some
     general rules for the decisions are:

     o  The client MAY check that the identity presented in the server's
        certificate matches the intended server hostname or domain.
        This check is not required (and may fail in the absence of the
        TLS server_name extension [TLS-EXT], as described above), but if
        it is implemented and the match fails, the client SHOULD either
        request explicit user confirmation, or terminate the connection
        but allow the user to disable the check in the future.
     o  Generally an NNTP server would want to accept any verifiable
        certificate from a client, however authentication can be done
        using the client certificate (perhaps in combination with the
        SASL EXTERNAL mechanism [NNTP-AUTH], although an implementation
        supporting STARTTLS is not required to support SASL in general
        or that mechanism in particular).  The server MAY use



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        information about the client certificate for identification of
        connections or posted articles (either in its logs or directly
        in posted articles).

2.2.2.2. Result of the STARTTLS Command

     Upon successful completion of the TLS handshake, the NNTP protocol
     is reset to the state immediately after the initial greeting
     response (see 5.1 of [NNTP]) has been sent, with the exception that
     if a MODE READER command has been issued, the effects of it (if
     any) are not reversed.  In this case, as no greeting is sent, the
     next step is for the client to send a command.  The server MUST
     discard any knowledge obtained from the client, such as the current
     newsgroup and article number, that was not obtained from the TLS
     negotiation itself.  Likewise, the client SHOULD discard and MUST
     NOT rely on any knowledge obtained from the server, such as the
     capability list, which was not obtained from the TLS negotiation
     itself.

     Both the client and the server MUST know if there is a TLS session
     active.  A client MUST NOT attempt to start a TLS session if a TLS
     session is already active.  A server MUST NOT return the STARTTLS
     capability label in response to a CAPABILITIES command received
     after a TLS handshake has completed, and a server MUST respond with
     a 502 response code if a STARTTLS command is received while a TLS
     session is already active.  Additionally, per section 3.4.2 of
     [NNTP], the client MUST NOT issue a MODE READER command while a TLS
     session is active and a server MUST NOT advertise the MODE-READER
     capability.

     The capability list returned in response to a CAPABILITIES command
     received after the TLS handshake MAY be different than the list
     returned before the TLS handshake.  For example, an NNTP server
     supporting SASL [NNTP-AUTH] might not want to advertise support for
     a particular mechanism unless a client has sent an appropriate
     client certificate during a TLS handshake.

2.2.3. Examples

     Example of a client being prompted to use encryption and
     negotiating it successfully (showing the removal of STARTTLS from
     the capability list once a TLS layer is active), followed by a
     successful selection of the group and an (inappropriate) attempt by
     the client to initiate another TLS negotiation:

        [C] CAPABILITIES
        [S] 101 Capability list:
        [S] VERSION 2



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        [S] READER
        [S] STARTTLS
        [S] LIST ACTIVE NEWSGROUPS
        [S] OVER
        [S] .
        [C] GROUP local.confidential
        [S] 483 Encryption or stronger authentication required
        [C] STARTTLS
        [S] 382 Continue with TLS negotiation
        [TLS negotiation occurs here]
        [Following successful negotiation, traffic is via the TLS layer]
        [C] CAPABILITIES
        [S] 101 Capability list:
        [S] VERSION 2
        [S] READER
        [S] LIST ACTIVE NEWSGROUPS
        [S] OVER
        [S] .
        [C] GROUP local.confidential
        [S] 211 1234 3000234 3002322 local.confidential
        [C] STARTTLS
        [S] 502 STARTTLS not allowed with active TLS layer

     Example of a request to begin TLS negotiation declined by the
     server:

        [C] STARTTLS
        [S] 580 Can not initiate TLS negotiation

     Example of a failed attempt to negotiate TLS, followed by two
     attempts at selecting groups only available under a security layer
     (in the first case the server allows the session to continue, in
     the second it closes the connection).  Note that unrestricted
     commands such as CAPABILITIES are unaffected by the failure:

        [C] STARTTLS
        [S] 382 Continue with TLS negotiation
        [TLS negotiation is attempted here]
        [Following failed negotiation, traffic resumes without TLS]
        [C] CAPABILITIES
        [S] 101 Capability list:
        [S] VERSION 2
        [S] READER
        [S] STARTTLS
        [S] LIST ACTIVE NEWSGROUPS
        [S] OVER
        [S] .
        [C] GROUP local.confidential



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        [S] 483 Encryption or stronger authentication required
        [C] GROUP local.private
        [S] 400 Closing connection due to lack of security

3. Augmented BNF Syntax for the STARTTLS Extension

     This section describes the syntax of the STARTTLS extension.  It
     extends the syntax in [NNTP], and non-terminals not defined in this
     document are defined there.

3.1. Commands

     This syntax extends the non-terminal "command", which represents an
     NNTP command.

     command =/ starttls-command

     starttls-command = "STARTTLS"

3.2. Capability entries

     This syntax extends the non-terminal "capability-entry", which
     represents a capability that may be advertised by the server.

     capability-entry =/ starttls-capability
     starttls-capability = "STARTTLS"

4. Summary of Response Codes

     This section contains a list of every new response code defined in
     this document, whether it is multi-line, which commands can
     generate it, what arguments it has, and what its meaning is.

     Response code 382
        Generated by: STARTTLS
        Meaning: continue with TLS negotiation

     Response code 580
        Generated by: STARTTLS
        Meaning: can not initiate TLS negotiation

5. Security Considerations

     In general, the security considerations of the TLS protocol [TLS]
     and any implemented extensions [TLS-EXT] are applicable here; only
     the most important are highlighted specifically below.  Also, this
     extension is not intended to cure the security considerations
     described in section 11 of [NNTP]; those considerations remain



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     relevant to any NNTP implementation.

     Use of STARTTLS cannot protect protocol exchanges conducted prior
     to authentication.  For this reason, the CAPABILITIES command
     SHOULD be re-issued after successful negotiation of a security
     layer, and other protocol state SHOULD be re-negotiated as well.

     It should be noted that NNTP is not an end-to-end mechanism. Thus,
     if an NNTP client/server pair decide to add TLS privacy, they are
     securing the transport only for that link.  Similarly, because
     delivery of a single piece of news may go between more than two
     NNTP servers, adding TLS privacy to one pair of servers does not
     mean that the entire NNTP chain has been made private.
     Furthermore, just because an NNTP server can authenticate an NNTP
     client, it does not mean that the articles from the NNTP client
     were authenticated by the NNTP client when the client received
     them.

     Both the NNTP client and server must check the result of the TLS
     negotiation to see whether an acceptable degree of authentication
     and privacy was achieved.  Ignoring this step completely
     invalidates using TLS for security.  The decision about whether
     acceptable authentication or privacy was achieved is made locally,
     is implementation-dependent, and is beyond the scope of this
     document.

     The NNTP client and server should note carefully the result of the
     TLS negotiation.  If the negotiation results in no privacy, or if
     it results in privacy using algorithms or key lengths that are
     deemed not strong enough, or if the authentication is not good
     enough for either party, the client may choose to end the NNTP
     session with an immediate QUIT command, or the server may choose
     not to accept any more NNTP commands.

     The client and server should also be aware that the TLS protocol
     permits privacy and security capabilities to be renegotiated mid-
     connection (see section 7.4.1 of [TLS]).  For example, one of the
     parties may desire minimal encryption after any authentication
     steps have been performed.  This underscores the fact that security
     is not present simply because TLS has been negotiated; the nature
     of the established security layer must be considered.

     A man-in-the-middle attack can be launched by deleting the STARTTLS
     capability label in the CAPABILITIES response from the server.
     This would cause the client not to try to start a TLS session.
     Another man-in-the-middle attack is to allow the server to announce
     its STARTTLS capability, but to alter the client's request to start
     TLS and the server's response.  An NNTP client can partially



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     protect against these attacks by recording the fact that a
     particular NNTP server offers TLS during one session and generating
     an alarm if it does not appear in the CAPABILITIES response for a
     later session (of course, the STARTTLS capability would not be
     listed after a security layer is in place).

     If the TLS negotiation fails or if the client receives a 483
     response, the client has to decide what to do next.  The client has
     to choose among three main options:  to go ahead with the rest of
     the NNTP session, to retry TLS later in the session, or to give up
     and postpone newsreading activity.  If a failure or error occurs,
     the client can assume that the server may be able to negotiate TLS
     in the future, and should try to negotiate TLS in a later session.
     However, if the client and server were only using TLS for
     authentication and no previous 480 response was received, the
     client may want to proceed with the NNTP session, in case some of
     the operations the client wanted to perform are accepted by the
     server even if the client is unauthenticated.

     Before the TLS handshake has begun, any protocol interactions are
     performed in the clear and may be modified by an active attacker.
     For this reason, clients and servers MUST discard any sensitive
     knowledge obtained prior to the start of the TLS handshake upon
     completion of the TLS handshake.

6. IANA Considerations

     This section gives a formal definition of the STARTTLS extension as
     required by Section 3.3.4 of [NNTP] for the IANA registry.

     o  The STARTTLS extension provides connection-based security via
        the Transport Layer Security (TLS).

     o  The capability label for this extension is "STARTTLS".

     o  The capability label has no arguments.

     o  This extension defines one new command, STARTTLS, whose
        behaviour, arguments, and responses are defined in Section 2.2.

     o  This extension does not associate any new responses with pre-
        existing NNTP commands.

     o  This extension does affect the overall behaviour of both server
        and client, in that after successful use of the STARTTLS
        command, all communication is transmitted with the TLS layer as
        an intermediary.




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     o  This extension does not affect the maximum length of commands or
        initial response lines.

     o  This extension does not alter pipelining, but the STARTTLS
        command cannot be pipelined.

     o  Use of this extension does alter the capabilities list; once the
        STARTTLS command has been used successfully, the STARTTLS
        capability can no longer be advertised by CAPABILITIES.
        Additionally, the MODE-READER capability MUST NOT be advertised
        after a successful TLS negotiation (as discussed in Section
        3.4.2 of [NNTP]).

     o  This extension does not cause any pre-existing command to
        produce a 401, 480, or 483 response.

     o  This extension is unaffected by any use of the MODE READER
        command, however the MODE READER command MUST NOT be used in the
        same session following a successful TLS negotiation (as
        discussed in Section 3.4.2 of [NNTP]).

     o  Published Specification: This document.

     o  Author, Change Controller, and Contact for Further Information:
        Author of this document.

7. References

7.1. Normative References

     [ABNF] Crocker, D., Overell, P., "Augmented BNF for Syntax
     Specifications:  ABNF", RFC 2234, November 1997.

     [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
     Requirement Levels", RFC 2119, March 1997.

     [NNTP] Feather, C., "Network News Transport Protocol",
     draft-ietf-nntpext-base-*.txt, Work in Progress.

     [TLS] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC 2246,
     January 1999.

     [TLS-EXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
     Wright, T., "Transport Layer Security (TLS) Extensions", RFC 3546, June
     2003.






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7.2. Informative References

     [NNTP-AUTH] Vinocur, J., Murchison, K., Newman, C., "NNTP Extension
     for Authentication", draft-ietf-nntpext-auth-*.txt, Work in
     Progress.

     [TLS-IMAPPOP] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
     2595, June 1999.

8. Authors' Addresses

     Jeffrey M. Vinocur
     Department of Computer Science
     Upson Hall
     Cornell University
     Ithaca, NY  14853

     EMail: vinocur@cs.cornell.edu


     Kenneth Murchison
     Oceana Matrix Ltd.
     21 Princeton Place
     Orchard Park, NY 14127 USA

     Email: ken@oceana.com


     Chris Newman
     Sun Microsystems
     1050 Lakes Drive, Suite 250
     West Covina, CA  91790

     EMail: cnewman@iplanet.com

9. Acknowledgements

     A significant amount of the STARTTLS text was lifted from RFC 3207
     by Paul Hoffman.

     Special acknowledgement goes also to the people who commented
     privately on intermediate revisions of this document, as well as
     the members of the IETF NNTP Working Group for continual insight in
     discussion.







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10. Intellectual Property Rights

     The IETF takes no position regarding the validity or scope of any
     intellectual property or other rights that might be claimed to
     pertain to the implementation or use of the technology described in
     this document or the extent to which any license under such rights
     might or might not be available; neither does it represent that it
     has made any effort to identify any such rights.  Information on
     the IETF's procedures with respect to rights in standards-track and
     standards-related documentation can be found in BCP-11.  Copies of
     claims of rights made available for publication and any assurances
     of licenses to be made available, or the result of an attempt made
     to obtain a general license or permission for the use of such
     proprietary rights by implementers or users of this specification
     can be obtained from the IETF Secretariat.

     The IETF invites any interested party to bring to its attention any
     copyrights, patents or patent applications, or other proprietary
     rights which may cover technology that may be required to practice
     this standard.  Please address the information to the IETF
     Executive Director.

11. Copyright

     Copyright (C) The Internet Society (2005).  This document is
     subject to the rights, licenses and restrictions contained in BCP
     78, and except as set forth therein, the authors retain all their
     rights."

     This document and the information contained herein are provided on
     an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
     REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
     THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
     EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
     THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
     ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
     PARTICULAR PURPOSE.














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